The subject matter disclosed herein relates to manufacturing of machine components comprising airfoil portions such as, but not limited to, rotor and stator blades or buckets for axial turbomachines, impellers for radial or axial-radial turbomachines and the like.
Axial turbomachines, such as axial compressors and turbines, comprise one or more stages, each stage being comprised of a circular arrangement of stationary blades or buckets and circular arrangement of rotor blades or buckets. The blades are provided with a root and a tip. An airfoil portion extends between the root and the tip of each blade.
In order to improve the turbomachine efficiency, the blades are usually subject to a polishing step. Additional treatments can be performed on the blades prior to polishing. For example a shot peening step is usually performed prior to polishing or finishing, for increasing the blade strength. Shot peening increases the surface roughness. The polishing step is currently performed by vibratory finishing, e.g. by vibro-tumbling. Vibro-tumbling provides for the blades to be placed in a rotating tumbler filled with pellets made of a natural abrasive or synthetic abrasive and a ceramic binder. The tumbler is caused to rotate and/or vibrate so that the pellets polish the surface of the airfoil profile. The final arithmetic average roughness (Ra) which can be achieved by vibro-tumbling ranges around 0.63 μm.
Lower roughness values could be achieved by continuing the vibro-tumbling treatment of the blades. However, the effect of the pellets on the airfoil profile not only modifies the surface roughness and texture, but also the airfoil geometry. Lowering the roughness below the abovementioned values would result in inadmissible alterations of the geometry. For this reason, lower roughness values cannot be obtained with the polishing methods of the current art
Shrouded impellers, e.g. for centrifugal compressors and pumps, are currently polished by means of so called abrasive flow machining. The abrasive flow machining process consists of generating a flow of a liquid suspension of abrasive material under pressure through the vanes of the impeller. Roughness values around 0.68 μm are achieved. Abrasive flow machining adversely affects the geometry of the blades, due to the abrasive action of the abrasive particles contained in the liquid suspension which is caused to flow under pressure through the vanes of the impeller. Moreover, the interaction between the blades and the abrasive flow is such that a non-homogeneous abrasive effect is obtained on the pressure side and suction side of each blade, due to the geometry of the latter. It is therefore not suitable to continue the abrasive flow machining process of an impeller beyond the above mentioned roughness values, since this would result in an unacceptable alteration of the blade geometry and therefore deterioration of the impeller efficiency.
The efficiency of a mechanical component comprised of an airfoil portion, such as an impeller or a blade, increases with reduced roughness, since energy losses due to friction are reduced. There is, therefore, a need for improving the finishing processes and methods in order to increase the efficiency of the airfoil profile by reducing the roughness thereof, without altering the geometry of the airfoil profile beyond an admissible threshold or tolerance.